Spectroscopic instruments are fairly well known. X-ray based instruments, for example, can be used to determine the elemental make up of a sample using x-ray fluorescence spectroscopy. Portable XRF has become a preferred technique for elemental analysis in the field. Portable XRF is fast, non-destructive, and provides reasonably accurate results (i.e., quantification of elemental concentrations in a wide variety of samples). With XRF, an x-ray tube is used to direct x-rays at a sample. Atoms in the sample absorb x-rays and re-emit x-rays that are unique to the atomic structure of a given element. A detector measures the energy of each x-ray and counts the total number of x-rays produced at a given energy. From this information, the types of elements and the concentration of each element can be deduced. Commercially available analyzers include the Delta manufactured by Olympus NDT and the Niton XLT-3 manufactured by Thermo Fisher Scientific.
X-rays, however, pose a safety concern. Also, portable and benchtop XRF analyzers have not to date been used to determine lower atomic number elements such as beryllium, sodium, carbon, magnesium, and the like.
Laser induced break down spectroscopy (LIBS) devices are known and used to detect the elemental concentration of lower atomic numbered elements with some accuracy. These devices typically include a high powered laser that sufficiently heats a portion of the sample to produce a plasma. As the plasma cools, eventually the electrons return to their ground states. In the process, photons are emitted at wavelengths unique to the specific elements comprising the sample. The photon detection and subsequent measurement of elemental concentrations are very similar to spark optical emission spectroscopy (OES). Examples of LIBS devices are the LIBS SCAN 25 from Applied Photonics, the LIBS25000 from Ocean Optics, and the RT 100 from Applied Spectra.
Still other instruments are better at determining the molecular compositions present in a sample. Portable, laser based Raman spectrometers or a wide bandwidth based (i.e., non-laser) near infra-red (NIR) analyzers can be used. These devices are configured to collect either Raman spectra or infra-red absorption from a given sample. They then compare the acquired spectra to a library of spectra of pure compounds. From the comparisons, the devices then determine the major compounds present in the sample. The process of determining what combination of pure compounds spectra in published libraries yield the measured spectrum of an unknown mixture is called chemometrics. There are several commercially available portable devices utilizing Raman technology including those manufactured by Thermo Fisher Scientific, Delta Nu and B&W Tek. For NIR, commercially available devices are made by ASD, Thermo Fisher Scientific, and Spectral Evolution.
Portable Raman and NIR analyzers are able to identify compounds present in a mixture, but they are generally limited to identifying what main compounds are present (as opposed to how much of each compound is present), or, at best, they can provide an approximate quantification of only a few components in a mixture of compounds. This limitation is due to sample response variation as a function of particle size, particle density, and mixture type, whether it be a solid solution or an inhomogeneous mixture of compounds. These parameters can cause the spectrum from one material to be enhanced or reduced relative to the other materials to a fair extent. In addition, both the Raman and NIR methods are sensitive to material very near the sample surface so that any variation is bulk vs. surface concentrations will be missed. Even without these effects, the ability to derive chemical constituents from mathematically combining spectra of pure compounds to simulate the unknown mixture spectrum rapidly degrades after the third compound, even with good quality spectra. In addition, currently available portable Raman and NIR units typically require a good deal of spectral interpretation from the operator, thus limiting user community to more technical users.
It is also known to fuse the data in dual source systems. That is, for example, Raman spectra data and LIBS spectra data are obtained and software is configured to calculate probability values to pinpoint an unknown material like a microorganism. See for example, published U.S. Patent Application Nos. 2009/0163369 and 2011/0080577 and U.S. Pat. No. 7,999,928 all incorporated herein by this reference.
Still, LIBS spectroscopy, for example, can produce inaccurate elemental concentrations in some cases and Raman and NIR spectroscopy can report one or more inaccurate compositions, mainly because for many compounds, the Raman or NIR spectra produced by those compounds are very similar. Plus, some libraries contain more than 10,000 spectra from the many compounds. Fusing the data may not improve accuracy.